This invention relates to a process for recovering lead from anode slime in order to upgrade the slime.
The presence of a large amount of lead in the slime makes the recovery of precious metals and other valuable metals from the slime more difficult. A variety of techniques have been utilized to recover the precious metals from the slime. None, however, have been commercially successful because of the large cost involved and the fact that it is difficult to recover the precious metals from the slime. One type of technique involves direct smelting of slimes. This is disadvantageous as excessive formation of matte and slag causes heavy recirculation of precious metals. Such techniques involve roasting to convert copper to copper oxide followed by leaching of the roasted slimes with sulfuric acid. The leached slimes are then melted down in a small reverberatory (dore) furnace in which the impurities are oxidized, leaving a dore bullion of the precious metals silver and gold. The dore is then pasted to recover the fine gold, fine silver and any other precious metal present in the original slime. This particular technique is, as noted, unsuitable because of the excessive formation of matte and slag.
A variety of other pyrometallurgical techniques combined with acid leachs and acid leach processes have also been attempted, but they are not suitable mainly because it has not been possible to initially remove substantially all of the lead from the slime. It has been found that the lead content of the slime cannot be removed by acid leaching. The presence of lead in the slime has greatly inhibited the known processes from recovering precious metals from slime.